Change default datetime format when converting to string - sql-server

Question

When a datetime is automatically converted to a string it uses this undesired format:
mon dd yyyy hh:miAM
how can I change this default so it is a more typical format like:
yyyy-mm-dd hh:mi:ss.mmm
Also, I don't want to have to explicitly use a cast/convert each time:
Convert(char(23),__date__,121)
I'd like to use CONCAT() without wrapping a convert around the datetimes. I generally keep appending a string variable by dumping in local variables, etc. as my stored procedures process. In the CATCH, I write this string out, but whenever datetime values are concatenated in, it uses the undesired format.
Sample code:
Declare #Now datetime=GETDATE()
,#Temp varchar(100)
SET #Temp=CONCAT('It is now: ',#now)
PRINT #Temp -----------------------It is now: Feb 8 2019 1:45PM
SELECT #Now -----------------------2019-02-08 13:45:05.247
Print #Now ------------------------Feb 8 2019 1:45PM
Print Convert(char(23),#Now,121)---2019-02-08 13:45:05.247
Output:
It is now: Feb 8 2019 1:45PM
-----------------------
2019-02-08 13:45:05.247
(1 row affected)
Feb 8 2019 1:45PM
2019-02-08 13:45:05.247

Answer 1

If you are writing new code, and you are ok with creating objects to support your desired format, then you can do it in a function.
CREATE FUNCTION dbo.fn_NewDate()
RETURNS varchar(23)
AS
BEGIN
RETURN(CONVERT(VARCHAR(23), GETDATE(), 121));
END
GO
Declare #Now varchar(23) = dbo.fn_NewDate()
,#Temp varchar(100);
SET #Temp=CONCAT('It is now: ', #now)
print #Temp;
Result:
It is now: 2019-02-08 12:43:38.980